学步园

主要思路就是求出序列的逆序数，此逆序数就是冒泡排序的交换次数

求逆序数采用归并排序，因为在每次归并排序中，如果左边大于右边，那么左边从此往后的数全部大于右边

code：

#include <set>
#include <map>
#include <ctime>
#include <queue>
#include <cmath>
#include <stack>
#include <limits>
#include <vector>
#include <bitset>
#include <string>
#include <cstdio>
#include <cstring>
#include <fstream>
#include <string.h>
#include <iostream>
#include <algorithm>
#define ls rt<<1
#define rs rt<<1|1
#define Si set<int>
#define LL long long
#define pb push_back
#define PS printf(" ")
#define Vi vector<int>
#define LN printf("\n")
#define SD(a) scanf("%d",&a)
#define PD(a) printf("%d",a)
#define SET(a,b) memset(a,b,sizeof(a))
#define FF(i,a) for(int i(0);i<(a);i++)
#define FD(i,a) for(int i(a);i>=(1);i--)
#define FOR(i,a,b) for(int i(a);i<=(b);i++)
#define FOD(i,a,b) for(int i(a);i>=(b);i--)
#define readf freopen("input.txt","r",stdin)
#define writef freopen("output.txt","w",stdout)
const int maxn = 500001;
const int INF = 0x3fffffff;
const int dx[]={0,1,0,-1};
const int dy[]={1,0,-1,0};
const double pi = acos(-1.0);
using namespace std;
int sr[maxn],tr[maxn];
__int64 cnt;
void Merge(int r, int m, int t){

	int k, j, i=r;
	for (j =m+1, k = i; i<=m && j <= t; ++k){
		if (sr[i] > sr[j]){
			tr[k] = sr[j++];
			cnt += m-i+1;
		}
		else
			tr[k] = sr[i++];
	}
	while (i<=m)
		tr[k++] = sr[i++];
	while (j<=t)
		tr[k++] = sr[j++];
	for (i=r; i<=t; ++i)
		sr[i] = tr[i];
}//Merge
void MSort(int r, int t){

	int m = (r + t)/2;
	if (r < t){
		MSort(r, m);
		MSort(m+1, t);
		Merge(r, m, t);
	}

}//MSort
int main(){
	int n;
	while (~scanf("%d", &n), n){
		int i;
		FOR(i,1,n) SD(sr[i]);
		cnt = 0;
		MSort(1,n);

		printf("%I64d\n", cnt);
	}
	return 0;
}